Loss of RPS41 but not its paralog RPS42 results in altered growth, filamentation and transcriptome changes in Candida albicans

A Small Molecule Inhibitor of Erg251 Makes Fluconazole Fungicidal by Inhibiting the Synthesis of the 14α-Methylsterols

Fluconazole (FLC) is widely used to prevent and treat invasive fungal infections. However, FLC is a fungistatic agent, allowing clinical FLC-susceptible isolates to tolerate FLC. Making FLC fungicidal in combination with adjuvants is a promising strategy to avoid FLC resistance and eliminate the persistence and recurrence of fungal infections. Here, we identify a new small molecule compound, CZ66, that can make FLC fungicidal. The mechanism of action of CZ66 is targeting the C-4 sterol methyl oxidase, encoded by the ERG251 gene, resulting in decreased content of sterols with the 14α-methyl group and ultimately eliminating FLC tolerance of Candida albicans. CZ66 most likely interacts with Erg251 through residues Glu195, Gly206, and Arg241. Establishing Erg251 as a synergistic lethal target protein of FLC should direct research to identify specific small molecule inhibitors of 14α-methylsterol synthesis and open the way to abolishing fungal FLC tolerance. IMPORTANCE Fluconazole (FLC) tolerance increases the frequency of acquired FLC resistance, and a high FLC tolerance level is associated with persistent candidemia. Multiple functional proteins, such as calcineurin, heat shock protein 90 (Hsp90), and ADP ribosylation factor, are essential for the survival of C. albicans exposed to FLC, but how these factors increase the fungicidal activity of FLC remains to be determined. In this study, we found that 14α-methylsterols replace ergosterol to allow C. albicans to survive FLC, but Erg251 inactivated by CZ66 results in loss of 14α-methylsterol synthesis and cell death of C. albicans treated with FLC. Establishing Erg251 as a synergistic lethal target protein of FLC should direct research to identify specific small molecule inhibitors of 14α-methylsterol synthesis and open the way to abolishing fungal FLC tolerance.

Comparative transcriptome analysis of Armillaria gallica 012m in response to ethephon treatment

Background

Gastrodia elata, known as a rootless, leafless, achlorophyllous and fully mycoheterotrophic orchid, needs to establish symbionts with particular Armillaria species to acquire nutrition and energy. Previous research findings had approved that ethylene (ET) played an important role in plant-fungi interaction and some receptors of ET had been discovered in microorganisms. However, the molecular mechanisms underlying the role of ET in the interaction between G. elata and Armillaria species remain unknown.

Methods

Exiguous ethephon (ETH) was added to agar and liquid media to observe the morphological features of mycelium and count the biomass respectively. Mycelium cultured in liquid media with exiguous ETH (0.1 ppm, 2.0 ppm, 5.0 ppm) were chosen to perform whole-transcriptome profiling through the RNA-seq technology (Illumina NGS sequencing). The DEGs of growth-related genes and candidate ET receptor domains were predicted on SMART.

Results

ETH-0.1 ppm and ETH-2 ppm could significantly improve the mycelium growth of A. gallica 012m, while ETH-5 ppm inhibited the mycelium growth in both solid and liquid media. The number of up-regulated or down-regulated genes increased along with the concentrations of ETH. The growth of mycelia might benefit from the up-regulated expression of Pyr_redox (Pyridine nucleotide-disulphide oxidoreductase), GAL4 (C6 zinc finger) and HMG (High Mobility Group) genes in the ETH-0.1 ppm and ETH-2 ppm. Therefore, the growth of mycelia might be impaired by the down-regulated expression of ZnF_C2H2 and ribosomal protein S4 proteins in the ETH-5 ppm. Seven ET receptor domains were predicted in A. gallica 012m. Based on cluster analysis and comparative studies of proteins, the putative ETH receptor domains of A. gallica 012m have a higher homologous correlation with fungi.

Conclusions

The responses of A. gallica 012m to ETH had a concentration effect similar to the plants’ responses to ET. Therefore, the number of up-regulated or down-regulated genes are increased along with the concentrations of ETH. Seven ET receptor protein domains were predicted in the genome and transcriptome of A. gallica 012m. We speculate that ETH receptors exist in A. gallica 012m and ethylene might play an important role in the plant-fungi interaction.

Myriocin enhances the antifungal activity of fluconazole by blocking the membrane localization of the efflux pump Cdr1

Introduction: Extrusion of azoles from the cell, mediated by an efflux pump Cdr1, is one of the most frequently used strategies for developing azole resistance in pathogenic fungi. The efflux pump Cdr1 is predominantly localized in lipid rafts within the plasma membrane, and its localization is sensitive to changes in the composition of lipid rafts. Our previous study found that the calcineurin signal pathway is important in transferring sphingolipids from the inner to the outer membrane. Methods: We investigated multiple factors that enhance the antifungal activity of fluconazole (FLC) using minimum inhibitory concentration (MIC) assays and disk diffusion assays. We studied the mechanism of action of myriocin through qRT-PCR analysis and confocal microscopy analysis. We tested whether myriocin enhanced the antifungal activity of FLC and held therapeutic potential using a mouse infection model. Results: We found that this signal pathway has no function in the activity of Cdr1. We found that inhibiting sphingolipid biosynthesis by myriocin remarkably increased the antifungal activity of FLC with a broad antifungal spectrum and held therapeutic potential. We further found that myriocin potently enhances the antifungal activity of FLC against C. albicans by blocking membrane localization of the Cdr1 rather than repressing the expression of Cdr1. In addition, we found that myriocin enhanced the antifungal activity of FLC and held therapeutic potential. Discussion: Our study demonstrated that blocking the membrane location and inactivating Cdr1 by inhibiting sphingolipids biogenesis is beneficial for enhancing the antifungal activity of azoles against azole-resistant C. albicans due to Cdr1 activation.

Baicalein Acts against Candida albicans by Targeting Eno1 and Inhibiting Glycolysis

Baicalein (BE) is a promising antifungal small-molecule compound with an extended antifungal spectrum, good synergy with fluconazole, and low toxicity, but its target protein and antifungal mechanism remain elusive. In this study, we found that BE can function against Candida albicans by disrupting glycolysis through targeting Eno1 and inhibiting its function. Eno1 acts as a key therapeutic target of the drug, as BE had no antifungal activity against the eno1 null mutant in a Galleria mellonella model of C. albicans infection. To investigate the mechanism of action, we solved the crystal structure of C. albicans Eno1(CaEno1) and then compared the difference between this structure and that of Eno1 from humans. The predicted primary binding site of BE on CaEno1 is between amino acids D261 and W274, with D263, S269, and K273 playing critical roles in the interaction with BE. Both positions S269 and K273 have different residues in the human Eno1 (hEno1). This finding suggests that BE may bind selectively to CaEno1, which would limit the potential for side effects in humans. Our findings demonstrate that Eno1 is a target protein of BE and thus may serve as a novel target for the development of antifungal therapeutics acting through the inhibition of glycolysis.

IMPORTANCE Baicalein (BE) is a promising antifungal agent which has been well characterized, but its target protein is still undiscovered. The protein Eno1 plays a crucial role in the survival of Candida albicans. However, there are few antifungal agents which inhibit the functions of Eno1. Here, we found that BE can function against Candida albicans by disrupting glycolysis through targeting Eno1 and inhibiting its function. We further solved the crystal structure of C. albicans Eno1(CaEno1) and predicted that the primary binding site of BE on CaEno1 is between amino acids D261 and W274, with D263, S269, and K273 playing critical roles in the interaction with BE. Our findings will be helpful to get specific small-molecule inhibitors of CaEno1 and open the way for the development of new antifungal therapeutics targeted at inhibiting glycolysis.

A comprehensive analysis of Candida albicans phosphoproteome reveals dynamic changes in phosphoprotein abundance during hyphal morphogenesis

The morphological plasticity of Candida albicans is a virulence determinant as the hyphal form has significant roles in the infection process. Recently, phosphoregulation of proteins through phosphorylation and dephosphorylation events has gained importance in studying the regulation of pathogenicity at the molecular level. To understand the importance of phosphorylation in hyphal morphogenesis, global analysis of the phosphoproteome was performed after hyphal induction with elevated temperature, serum, and N-acetyl-glucosamine (GlcNAc) treatments. The study identified 60, 20, and 53 phosphoproteins unique to elevated temperature-, serum-, and GlcNAc-treated conditions, respectively. Distribution of unique phosphorylation sites sorted by the modified amino acids revealed that predominant phosphorylation occurs in serine, followed by threonine and tyrosine residues in all the datasets. However, the frequency distribution of phosphorylation sites in the proteins varied with treatment conditions. Further, interaction network-based functional annotation of protein kinases of C. albicans as well as identified phosphoproteins was performed, which demonstrated the interaction of kinases with phosphoproteins during filamentous growth. Altogether, the present findings will serve as a base for further functional studies in the aspects of protein kinase-target protein interaction in effectuating phosphorylation of target proteins, and delineating the downstream signaling networks linked to virulence characteristics of C. albicans.

Roles of RPS41 in Biofilm Formation, Virulence, and Hydrogen Peroxide Sensitivity in Candida albicans

In eukaryotes, loss of cytoplasmic ribosomal proteins (RPs) results in a reduced growth rate and other phenotypic defects. The ability to transition from a unicellular budding yeast to a filamentous form is very important for biofilm formation and virulence in Candida albicans. Our recent study found that loss of the RPS41 (C2_10620W_A) gene but not its paralog RPS42 (C1_01640W_A) resulted in altered growth and filamentation changes in C. albicans, so we hypothesized that the RPS41 gene should play important roles in virulence and biofilm formation in this pathogen. We found that both virulence and the ability to form biofilms were defective due to deletion of the RPS41 gene. We also found that loss of the RPS41 gene increased sensitivity to hydrogen peroxide, and that hydrogen peroxide induced the expression of the RPS41 gene in a wild-type strain. These results suggested that the RPS41 gene plays important roles in C. albicans biofilm formation, virulence, and susceptibility to hydrogen peroxide.